1. Field of the Invention
This invention relates to the detection and analysis of blood vessels, and is of particular application in the analysis of blood vessels in images of the ocular fundus (the back of the eye), hereinafter referred to as the fundus. The analysis of blood vessels in the fundus is important because the fundus is one of the few places in the body where blood vessels can be easily observed, and such observation is therefore a very convenient way of monitoring certain diseases associated with circulation defects, in particular arteriosclerosis, hypertension and diabetes.
2. Description of Background Art
Particular pathological manifestations which it is useful to monitor are features such as venous beading, focal arteriolar narrowing and generalized arteriolar narrowing. Measurement of these quantities requires very precise measurements of the caliber of the blood vessels. Hence, a precise knowledge of both localization and orientations of the vessels is important. Additional features, which are of interest in fundus images are micro-aneurysms and exudates, which show up on fundus images as generally “dot shaped” (i.e., circular, or approximately circular) areas. It is also of interest to be able to distinguish between such micro-aneurysms and exudates, and to distinguish them from other “dot shaped” pathologies in the image, e.g., so-called “cotton wool spots” and hemorrhages. In particular, it is desirable to determine the position of the dot shaped pathology relative to the vascular structure.
Currently, examination of fundus images is carried out principally by a clinician examining each image “manually”. This is not only very time-consuming, since even an experienced clinician can take several minutes to assess a single image, but is also prone to error since there can be inconsistencies between the way in which different clinicians assess a given image.
It is therefore desirable to provide ways of automating the process of the analysis of fundus images, using computerized image analysis, so as to provide at least preliminary screening information and also as an aid to diagnosis to assist the clinician in the analysis of difficult cases.
A number of steps are necessary to carry out such analysis. In general, it is necessary first to segment the fundus image into blood vessels and background. Visually, vessels in the fundus image appear as dark lines on a relatively uniform bright background. Various methods are known for segmenting the fundus image. Reference is made, for Example to the following:—                Tolias y a et al “A fuzzy vessel tracking algorithm for retinal images based on fuzzy clustering, IEEE Transactions On Medical Imaging, April 1998, IEEE, USA, vol. 17, no. 2, pages 263–273, ISSN: 0278–0062;        Akita k et al: “A computer method of understanding ocular fundus images” Pattern Recognition, 1982, UK, vol. 15, no. 6, pages 431–443, ISSN: 0031-3203 chapter 4;        Giansanti r et al: “Imaging system for retinal change evaluation” Sixth International Conference on Image Processing and its Applications (conf. publ. NO. 443), Proceedings of 6th International Conference on Image Processing and its Applications (conf publ. no. 443), Dublin, Ireland, 14–17 July 1997, pages 530–534 vol. 2, 1997, London, UK, IEE, UK ISBN: 0-85296-692-X chapter 2; and        Kurokawa T et al: “Maze-tracing algorithm applied to eye-fundus blood vessels” Electronics Letters, 14 MAY 1998, IEE, UK, vol. 34, no. 10, pages 976–977, ISSN: 0013-5194.        
Next, it is generally desirable to provide a method of determining accurately, using computerized image analysis techniques, the position of both the papilla (the point of exit of the optic nerve) and the fovea (the region at the center of the retina, where the retina is most sensitive to light).
Having identified the blood vessels in the image, it is desirable to be able to distinguish between veins and arteries among the blood vessels. This can be important, for example in the diagnosis of venous beading and focal arteriolar narrowing.
The vascular system observed in the ocular fundus images is by nature a 2-dimensional projection of a 3dimensional structure. It is quite difficult in principle to distinguish veins from arteries, solely by looking at isolated vessel segments. However, we have discovered that effective separation can be achieved by making use of the fact that, individually, the artery structure and the vein vessel structures is each a perfect tree, (i.e., there is one unique path along the vessels from the heart to each capillary and back).
On the retina, the artery and vein structures are each surface filling, so that all tissue is either supplied or drained by specific arteries or veins, respectively.